1. Field of the Invention
The present invention relates to a method of driving a piezoelectric actuator and a liquid ejection apparatus, and more particularly, to a method of driving a piezoelectric actuator in which an upper electrode film, a piezoelectric film and a lower electrode film are layered successively onto a diaphragm, and to a liquid ejection apparatus which includes the piezoelectric actuator.
2. Description of the Related Art
A piezoelectric actuator having a piezoelectric film provided between an upper electrode film and a lower electrode film has been known in the related art and has been used, for example, in an inkjet recording type of recording head (inkjet head). In general, this piezoelectric actuator has a structure in which a lower electrode film, a piezoelectric film and an upper electrode film are layered successively onto a diaphragm which forms one wall of a pressure chamber, and when an electric field is applied to the piezoelectric film, the piezoelectric film expands or contracts in the direction perpendicular to the thickness direction, causing the diaphragm to deform in a convex shape toward the side of the pressure chamber, and consequently the ink inside the pressure chamber is pressurized, and an ink droplet is ejected from a nozzle connected to the pressure chamber.
In recent years, there has been growing demand for higher quality in the recorded images produced by inkjet recording methods, and one of the technical issues involved in this is enhancing the performance and reliability of piezoelectric actuators. In order to resolve this issue, various technologies have been proposed thus far.
Japanese Patent Application Publication No. 2003-188429 describes a method of forming a crystalline piezoelectric body in which the crystalline structure and the preferentially-oriented plane are controlled by a sputtering technique, or the like, directly after depositing a thin film of piezoelectric material, without carrying out a crystallization process involving heat treatment.
Japanese Patent Application Publication No. 2003-260793 discloses a method for applying to a piezoelectric element an ejection drive signal having an electric potential raising step in which, after an initial electric potential which is a negative potential that is lower than the coercive electric field of the piezoelectric body, the potential is changed continuously to a positive potential which compresses the pressure chamber and causes an ink droplet to be ejected.
A piezoelectric film deposited by a sputtering technique, or the like has an orientation direction, and the orientation direction (the direction from the lower electrode film toward the upper electrode film) is determined at the time of film formation. Such a piezoelectric film produces an amount of displacement that is approximately proportional to the intensity of the applied electric field when an electric field is applied in the same direction as the direction of orientation of the piezoelectric film. On the other hand, when an electric field is applied in the opposite direction to the direction of orientation of the piezoelectric film, the displacement direction of the piezoelectric film is inverted around the coercive electric field of the piezoelectric film. Consequently, in a piezoelectric actuator comprising a piezoelectric film formed by a sputtering technique, or the like, it is common to adopt a composition in which the lower electrode film is used as an individual electrode (address electrode) and the upper electrode film is used as a common electrode (ground electrode), in order to apply an electric field to the piezoelectric film in the same direction as the direction of orientation of the piezoelectric film. This is because, if the upper electrode film is used as an individual electrode, then a negative voltage is required to be supplied to the upper electrode film, and the costs relating to the driver IC and so on increase in comparison with a case where a positive voltage is supplied.
On the other hand, if the lower electrode film is used as an individual electrode, while the upper electrode film is used as a common electrode, and if the diaphragm is made of a silicon substrate, for instance, then a current leakage occurs between the plurality of lower electrode films (individual electrodes), via the diaphragm, and a problem of electrical cross-talk arises, namely, ink droplets are ejected from nozzles where they are not intended to be ejected. Moreover, due to the increase in the electrostatic capacitance, there is also a drawback in that the power consumption increases.
Furthermore, in order to be able to eject very fine liquid droplets in an inkjet recording system, it is necessary to carry out ink ejection by means of a so-called “pull-push” operation, which involves continuously performing an operation of pulling the meniscus inside the nozzle and an operation of pushing the meniscus out from the nozzle. However, in order to achieve ink ejection by means of a “pull-push” operation simply by applying an electric field to the piezoelectric film in one direction only, the drive voltage waveform applied to the piezoelectric actuator becomes complex since, for example, the pressure chamber must first be contracted, and then returned provisionally to its original state and then contracted once again, and this leads to increased costs.
In the method described in Japanese Patent Application Publication No. 2003-188429, the direction of orientation of the piezoelectric body is the direction from the lower electrode toward the upper electrode, and if the upper electrode is used as an individual electrode, then in order to obtain a positive displacement (a displacement toward the side of the pressure chamber), it is necessary to apply a negative voltage to the upper electrode and as described above, this gives rise to increased costs in relation to the drive IC and the power supply, etc. On the other hand, if the lower electrode is used as the individual electrode, then problems of electrical cross-talk arise.
Furthermore, in the method described in Japanese Patent Application Publication No. 2003-260793, it is necessary to apply an electric field in both the same direction and the reverse direction of the direction of orientation of the piezoelectric body. In other words, both a positive and a negative voltage must be applied to the individual electrode (upper electrode or lower electrode), and therefore, similarly to the case of supplying a negative voltage, there is an increase in the costs relating to the drive IC and the like.